U.S. Patent Application Publication No. 2009/0046030 (PTL 1), International Publication No. 2009/033510 pamphlet (PTL 2), and U.S. Patent Application Publication No. 2004/0217909(PTL 3) disclose techniques for sensing surrounding conditions to compensate, by feedback control, antenna characteristics that change in accordance with the surrounding conditions such as an approaching human body.
In PTL 1, variation in the input impedance (return loss, VSWR) of an antenna caused by a change in surrounding conditions is directly measured. By providing a directional coupler on a line between an RF circuit and the antenna, power transmitted in a direction (input direction) from the RF circuit to the antenna and power transmitted in a direction (reflection direction) from the antenna to the RF circuit are monitored, thereby keeping track of the input impedance in the current state.
In PTL 2, a change in the strength of an electromagnetic wave radiated from an antenna is directly measured. A change in an electric field radiated from the antenna due to the effects of the surroundings is detected using a sensor such as a Hall element. Although the factors responsible for the change cannot be divided into a change in input impedance and an increased loss caused by absorption of the radiated electromagnetic wave by a nearby object, an overall change in the radiated electric field caused by these factors is detected.
In PTL 3, a distance to an approaching human body is measured. By using a light emitting/receiving device, a distance is measured on the basis of light reflected by a nearby object.
Here, the configuration of an antenna apparatus disclosed in PTL 2 is described with reference to FIG. 1.
The antenna apparatus is formed of an antenna 18, a high-frequency circuit 26 that inputs a radio frequency signal to the antenna 18, and several network components, and includes a first matching circuit 24 provided on a signal line between the antenna 18 and the high-frequency circuit 26, a controller 28, and a detector 10 that detects an electromagnetic field radiated from the antenna 18. The controller 28 performs matching control of the antenna 18 on the basis of a detected electric field.